1. Field of the Invention
The present invention relates to a method and an apparatus for processing a three-dimensional structure, a method for producing a product having a three-dimensional shape, and a three-dimensional structure, wherein a fine structure can be formed directly on a material to be processed by irradiating a laser beam, and more specifically to a method and an apparatus for processing a three-dimensional structure, a method for producing a product having a three-dimensional shape, and a three-dimensional structure, wherein a three-dimensional structure with a smooth flat bottom surface can be formed in a high controllability with respect to the process, using one-shot laser light pulses.
In particular, the object of the present invention is to produce in a high precision a product which requires to form a very fine structure, and thereby the present invention relates to a product having a fine three-dimensional shape, in which case, a method for forming recording pits in an optical disk, a method for forming a stamper for producing an original or master form for such an optical disk, a method for forming an optical element, such as a multi-level diffraction grating, a diffraction hologram, or the like, a method for forming an original or master form of such an optical element, and/or a method for processing a three-dimensional structure in a micro-machine, micro-sensor or the like, for processing a micro-structure is applied.
2. Description of the Related Art
Conventionally, the photolithography, together with the etching process, is exclusively employed to process a fine three-dimensional shape product with a high precision. In this case, a desired pattern is formed on a resist material by selectively irradiating light thereto and subsequently by applying a solution process to the resist material, and then a material to be processed is etched after the resist material thus processed is applied thereto, so that the etching is selectively carried out for only the surface onto which no resist material is covered. That is, the two areas, a part to be processed and another part not to be processed, are formed a series of processes, i.e., the application of the resist material, exposure, fixation, development, etching of the material to be processed and removing the resist material. In the case of processing a three-dimensional shape product, furthermore, a resist material is newly applied to the material to be processed, and then the above process is repeated after precisely adjusting the position of the material to be processed.
The etching method in the conventional photolithography requires a number of complicated processes, such as resist application, exposure, development, baking and so on. In the exposure process, the intensity (and time) of the exposure light has to be controlled in a precise and uniform manner in order to avoid a change in the resist pattern due to the variation of the exposure intensity.
In the case of a three-dimensional process where the depth is altered in accordance with the position, the shape has to be controlled using a number of expensive masks.
Moreover, in the case of controlling the depth for each position, an adjustment of precisely positioning the formed substrate is always required. Furthermore, the exposure condition is altered for a partially processed material to be processed, compared with that for a flat substrate.
Moreover, it is very difficult to uniformly apply a resist to a substrate due to the surface roughness, when it is partially processed in a three dimension.
As another example for a method of processing a three-dimensional shape in a fine element, a method using the laser process is known. In the process of metals using the conventional laser process method, a high power laser on the basis of the fundamental wave of CO2 or Nd:YAG laser is employed.
In recent years, the second or third harmonic of the YAG (Yttrium Aluminum Garnet), YLF (Yttrium Lithium Fluoride) or YVO4 (Yttrium Orthovanadate) is employed in order to realize a fine structure and a precise process.
As a laser light source used for fine process, an UV pulse laser, typically the excimer laser, is employed.
Such a laser normally has a wavelength of 157 nm to 309 nm and a pulse width of several ns to several tens ns. In particular, a polymer absorbent of light having such a wavelength may be processed to remove portions irradiated by such a laser using a pulse having a smaller width, compared with the thermal diffusion length. Therefore, this method is known as a process method providing a high precision without thermal damage.
In recent years, it is known that the femto-second laser is employed for a method of precisely processing a metal or the like. In this case, a laser having a pulse width of several tens femto-seconds to several hundred femto-seconds is used and Ti:sapphire is typically employed for its light source. It is known that this method is capable of providing a fine and precise process for various materials made of metal, ceramic or others. For instance, see the following papers by the present inventors, Kumagawa and Midorikawa: Appl. Phys. A 63, 109-115 (1996); Oyo-buturi (Jpn. J. Appl. Phys.) 67(9), 1051 (1998); and O Plus E 21 (9), 1130 (1999).
However, there are difficulties and drawbacks in the above-mentioned conventional laser process methods. For instance, the process using the CO2 or YAG laser is fundamentally based on the thermal process, so that it is difficult to process a polymer, low melting material or the like in a high precision, because the material to be processed in the vicinity of the area irradiated by the laser light is thermally deformed and/or melted. In the case of processing metals, thermally disturbed layers in the vicinity of the process area appear due to a high thermal conductivity, so that the deterioration of shape or profile in the area often occurs due to the melting, re-solidification and the like. In these cases, such thermal deterioration provides a reduction in the surface precision for the bottom surface of the processed material to be processed and melt marks on the surface.
Moreover, the application of the process using the excimer laser or the harmonic of the YAG laser is generally restricted to the materials having high absorption efficiency for such a laser wavelength due to the difficulty in the process. Actually, such a material pertains to a restricted type of polymers. In this laser process, it is also difficult to form the bottom surface of the material to be processed in a uniform height and flatness, and it is usually necessary to precisely control the beam shape of the laser light using an expensive optical system. In this case, it is necessary to process the material to be processed by projecting in the reduced mode the laser beam whose intensity is uniformed at the mask position by means of an optical element. Nevertheless, it is difficult to provide a three-dimensional process of the material to be processed having a uniform bottom surface due to the interference with the beams diffracted or split from the mask. It is particularly difficult to form a flat bottom surface in the precision of the order of several tens nm, which precision is required for producing optical elements.
It is known that a high precision can also be obtained even for a material to be processed of metal material with the aid of the abrasion process using the Ti:sapphire laser having a pulse width in a range of sub pico-seconds to pico-seconds. In this case, an expensive optics for forming a flat bottom surface must be provided, as similarly said abrasion is applied. In these lasers, the beam transverse mode is normally a single mode, and the beams diffracted in the mask are apt to interfere with each other. Moreover, there arise problems that the flatness of the processed surface is reduced due to speckle pattern and a fine period generates due to the polarization of the laser light, thereby making it difficult to produce flat surfaces in the material to be processed.
As for the process of a thin layer using a conventional laser, a metal removing method for correcting a photo-mask is known. In the metal removing method, a thin metal layer deposited onto a glass substrate is irradiated by a laser light to selectively remove parts of the layer by melt and evaporation.
In order to produce a three-dimensional structure, a method for laminating the layers of laser absorbing material is proposed in Japanese Patent Laid-Open Publication No. 10-223504. The other methods using a transparent material are proposed in Japanese Patent Laid-Open Publications No. 10-137953 and No. 10-319221. The former provides a method for removing a transparent thin layer of an electro-luminescence, in which case, an upper transparent layer is removed using the removing energy due to the laser abrasion. The latter relates to a method for producing a reflection type optical element, in which case, an anti-reflection film is removed by using the abrasion resulting from laser light passed through it.
Although the upper thin layer of the material to be processed can also be removed by the thin layer processing method using the conventional laser abrasion, no marked improvement in the surface roughness on the bottom surface of the material to be processed can be obtained, compared with that of the above-mentioned laser abrasion. In the case of only absorbent material being employed, a deep depth process can hardly be carried out by a one-shot laser radiation. In case of carrying out a deeper depth process, there is a problem that the quality of the finished material to be processed may be reduced due to the thermal effect. In the method of removing the upper layer using the absorbent layer of the bottom surface, the process of the absorbent layer also advances, so that it is difficult to control the depth in a three-dimensional manner or it is necessary to control the depth by several times irradiating the laser light. In any case, it is difficult to control the process of the three-dimensional shape available in an optical device in the precision of order of nano-meter.